Method of controlling wavelength-tunable laser

ABSTRACT

A method of controlling a wavelength-tunable laser selecting an oscillation wavelength with a combination of a plurality of wavelength selection portions of which wavelength peak is different from each other, comprising: a first step of confirming a control direction of the wavelength selection portion in a case where a setting value is changed from a first setting value for achieving the first wavelength to a second setting value for achieving the second wavelength; a second step of setting a setting value that is shifted from the second setting value in a direction that is opposite of a pre-determined changing direction on the wavelength selection portion as a prepared setting value, when the control direction confirmed in the first step is opposite to the pre-determined changing direction; and a third step of changing the prepared setting value set in the second step to the second setting value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of 13/029,539, filed Feb. 17, 2011,which is based upon and claims the benefit of priority of the priorJapanese Patent Application No. 2010-033273, filed on Feb. 18, 2010, theentire contents of which are incorporated herein by reference.

BACKGROUND

(i) Technical Field

The present invention relates to a method of controlling awavelength-tunable laser.

(ii) Related Art

Generally, a wavelength-tunable laser oscillates at a desirablewavelength by varying wavelength characteristics of a resonator.Typically, a mirror included in the resonator has wavelengthcharacteristics in order to vary the wavelength characteristics of theresonator. The wavelength characteristics of the mirror are controlledso that a resonant condition of the resonator fits a desirableoscillation wavelength. Another wavelength-tunable laser has a pluralityof ring resonators and controls an oscillation condition by controllingthe resonant condition. Japanese Patent Application Publication No.2007-48988 discloses the wavelength-tunable lasers.

A given setting value is provided to each part of the wavelength-tunablelaser in order to achieve a desirable oscillation wavelength. Forexample, a setting value is provided to a gain electrode for setting again of the resonator, a heater for controlling wavelengthcharacteristics of DBR (Distributed Reflector) region, and a temperaturecontrol device for controlling wavelength characteristics of a DFB(Distributed Feedback) region and a laser oscillation.

A number of wavelength channels is set in a narrow band in awavelength-tunable laser used in a WDM communication technology.Therefore, an interval of each channel is very narrow. For example, aC-band has 89 channels in a wavelength band from 1528.773 nm to 1563.863nm. Therefore, a frequency difference between each channel is 50 GHz(approximately 0.4 nm as wavelength difference).

The wavelength channel is defined with a narrow wavelength interval.However, the wavelength-tunable laser has a number of elements to becontrolled. Accordingly, only a narrow control range is allowed in asetting value of each element.

SUMMARY

It is an object of the present invention to provide a method ofcontrolling a wavelength-tunable laser that controls an oscillationwavelength with high controllability.

According to an aspect of the present invention, there is provided amethod of controlling a wavelength-tunable laser selecting anoscillation wavelength with a combination of a plurality of wavelengthselection portions of which wavelength peak is different from eachother, in a case where a setting value of at least one of the wavelengthselection portions is controlled in a process in which the oscillationwavelength is changed from a first wavelength to a second wavelength,comprising: a first step of confirming a control direction of thewavelength selection portion in a case where a setting value is changedfrom a first setting value for achieving the first wavelength to asecond setting value for achieving the second wavelength; a second stepof setting a setting value that is shifted from the second setting valuein a direction that is opposite of a pre-determined changing directionon the wavelength selection portion as a prepared setting value, whenthe control direction confirmed in the first step is opposite to thepre-determined changing direction; and a third step of changing theprepared setting value set in the second step to the second settingvalue.

According to another aspect of the present invention, there is provideda method of controlling a wavelength-tunable laser selecting anoscillation wavelength with a combination of a plurality of wavelengthselection portions of which wavelength peak is different from eachother, in a case where a setting value of at least one of the wavelengthselection portions is controlled in a process in which the oscillationwavelength is changed from a first wavelength to a second wavelength,comprising: a first step of changing a first setting value for achievingthe first wavelength set on the wavelength selection portion to aninitial setting value acting as a starting point of a control directiondetermined in advance; and a second step of changing the initial settingvalue set on the wavelength selection portion in the first step to asecond setting value for achieving the second wavelength.

According to another aspect of the present invention, there is provideda method of controlling a wavelength-tunable laser having a plurality ofwavelength selection portions and a gain region, the plurality of thewavelength selection portions, of which wavelength peak is differentfrom each other, selecting an oscillation wavelength with a combinationthereof, in a starting of the wavelength tunable laser, comprising: afirst step of setting an initial setting value for achieving a startingpoint of a control direction determined in advance on the wavelengthselection portion and setting a gain setting value for achieving apredetermined gain on the gain region; and a second step of setting atarget wavelength setting value for achieving a pre-determinedoscillation wavelength on the wavelength selection portion after thefirst step.

According to another aspect of the present invention, there is provideda method of controlling a wavelength-tunable laser having a plurality ofwavelength selection portions and a gain region, the plurality of thewavelength selection portions, of which wavelength peak is differentfrom each other, selecting an oscillation wavelength with a combinationthereof, in a starting of the wavelength tunable laser, comprising: afirst step of setting a target wavelength setting value for achieving apre-determined oscillation wavelength on the wavelength selectionportion and setting a gain setting value for achieving a gain on thegain region; a second step of setting an initial setting value acting asa starting point of a control direction determined in advance on thewavelength selection portion; and a third step of setting the targetwavelength setting value on the wavelength selection portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a wavelength-tunable laser;

FIGS. 2A and 2B illustrate a relationship between a temperaturecondition of a heater and an oscillation wavelength;

FIG. 3 illustrates a control flowchart for describing a firstembodiment;

FIG. 4 illustrates a setting value of a wavelength selection portion ina case where the control flowchart of FIG. 3 is executed;

FIG. 5A illustrates a control flowchart for describing a secondembodiment;

FIG. 5B illustrates a setting value of a wavelength selection portion ina case where the control flowchart of FIG. 5A is executed;

FIG. 6A illustrates a control flowchart for describing a thirdembodiment;

FIG. 6B illustrates a setting value of a wavelength selection portion ina case where the control flowchart of FIG. 6A is executed;

FIG. 7A illustrates a control flowchart for describing a fourthembodiment; and

FIG. 7B illustrates a setting value of a wavelength selection portion ina case where the control flowchart of FIG. 7A is executed.

DETAILED DESCRIPTION

A description will be given of a best mode for carrying the presentinvention.

First Embodiment

FIG. 1A illustrates an example of a wavelength-tunable laser 100 and alaser device 200 having the same. The wavelength-tunable laser 100 has astructure in which a SG-DFB (Sampled Grating Distributed Feedback)region 10 and a CSG-DBR (Chirped Sampled Grating Distributed Reflector)region 20 formed on a semiconductor substrate 1 are optically coupled toeach other. The wavelength-tunable laser 100 is provided on atemperature control device 50. The temperature control device 50 may usea peltier element.

The SG-DFB region 10 has a structure in which a corrugation 11 is formedat a given interval. The SG-DFB region 10 has a gain when a current isprovided to an active layer 12 from a gain electrode 40. The SG-DFBregion 10 has wavelength characteristics in which the active layer 12has gain peaks at a given wavelength interval, because the corrugation11 is formed at a given interval. Each interval of the corrugation 11 issubstantially the same in the SG-DFB region 10.

A corrugation 21 is formed at a given interval in the CSG-DBR region 20.The interval of the corrugation 21 is different from that of thecorrugation 11. At least two of the intervals of the corrugation 21 aredifferent from each other. An optical waveguide layer 22 of the CSG-DBRregion 20 has a reflection peak at a given interval that is differentfrom the gain peak of the active layer 12. The reflection intensity maybe different from each other in the CSG-DBR region 20. Heaters 30 athrough 30 c are provided on the CSG-DBR region 20 in a longitudinaldirection of the optical waveguide layer 22. The wavelengthcharacteristics of the CSG-DBR region 20 may be controlled with atemperature control of the heaters 30 a through 30 c.

A controller 60 controls the wavelength-tunable laser 100. Thecontroller 60 has a CPU (Central Processing Unit), a RAM (Random AccessMemory), an input/output interface and so on. The controller 60 sets asetting value on the gain electrode 40, the heaters 30 a through 30 cand the temperature control device 50. The setting value is set as anelectrical signal such as a current, a voltage or the like.

A driving current is provided to the gain electrode 40 so that a gainrequired for a desirable laser oscillation is achieved in a resonator.Each temperature of the heaters 30 a through 30 c is controlled so thata desirable oscillation wavelength is achieved. A memory 70 stores atemperature condition of the heaters 30 a through 30 c with respect toeach target oscillation wavelength. The controller 60 accesses thememory 70 based on target wavelength information and reads necessaryinformation. The temperature control device 50 controls the temperatureof the corrugations 11 formed at an interval and the active layer 12 andthereby controls refractive index of the SG-DFB region 10.

The controller 60 controls above-mentioned temperature with thetemperature control device 50 according to wavelength information thatis output from the wavelength-tunable laser 100 and is detected througha beam splitter 80 and a wavelength detection portion 90. Theoscillation wavelength is controlled with the temperature control of thetemperature control device 50.

The controller 60 controls the wavelength characteristics of the SG-DFBregion 10 of the wavelength-tunable laser 100 with use of the refractiveindex variation of the active layer 12 controlled with the temperaturecontrol of the temperature control device 50. The controller 60 controlsthe wavelength characteristics of the CSG-DBR region 20 of thewavelength-tunable laser 100 with use of the refractive index variationof the optical waveguide layer 22 controlled with the temperaturecontrol of the heaters 30 a through 30 c and the temperature controldevice 50.

With the structure, an overlap of the gain peak of the SG-DFB region 10and the reflection peak of the CSG-DBR region 20 may be controlled, anda desirable oscillation wavelength may be selected.

FIG. 2A illustrates a correlation of an oscillation wavelength of thewavelength-tunable laser 100 in a case where each temperature differencebetween the heaters 30 a through 30 c provided on the CSG-DBR region 20of the wavelength-tunable laser 100 is kept constant and the temperatureof the heaters 30 a through 30 c is varied. The variation of thetemperature of the heaters 30 a through 30 c corresponds to a selectionof the oscillation wavelength of the wavelength-tunable laser 100.

As illustrated in FIG. 2A, the oscillation wavelength skips with respectto the temperature variation of the heaters 30 a through 30 c. Acombination of the wavelength characteristics of the SG-DFB region 10and the CSG-DBR region 20 has an interval of a wavelength peak. Here, anarea where an oscillation wavelength is kept substantially constant withrespect to the temperature variation of the heaters 30 a through 30 c ishereinafter referred to as a terrace.

As illustrated in FIG. 2A, a part of the terraces adjacent to each otheroverlaps with each other. That is, two wavelengths may be selected at aspecific temperature condition. This means when the wavelength-tunablelaser 100 oscillates at a wavelength, a temperature of the heaters hasto be changed dynamically to another temperature of the heaters whereoscillation wavelengths does not overlap with each other, in order tochange the oscillation wavelength to an adjacent one. When a laseroscillation is achieved once, a carrier is induced to the oscillationcondition. Accordingly, the oscillation condition is not easily changedto the adjacent one. Thus, one oscillation wavelength overlaps withanother one. The overlapping is observed in other wavelength-tunablelasers.

The overlapping may cause difficulty of the temperature of the heaterssetting to achieve each oscillation wavelength. As mentioned above, thetemperature of the heaters 30 a through 30 c is determined by tuningwith respect to each oscillation wavelength according to each wavelengthchannel. Here, as illustrated in FIG. 2A, it is assumed that thetemperature of the heaters achieving oscillation wavelengths adjacent toeach other are determined a condition “a” and a condition “b”. Thecondition “a” is a temperature condition where the oscillationwavelength α overlaps with the oscillation wavelength β. The condition“b” is a temperature condition where the oscillation wavelength β doesnot overlap with another oscillation wavelength.

For example, electrical power according to the condition “b” has only tobe provided in order to change an oscillation wavelength to theoscillation wavelength β, when electrical power of the heaters accordingto the condition “a” is provided in a condition that the oscillation isachieved at the oscillation wavelength α. However, the oscillationwavelength is kept at the oscillation wavelength β, even if theelectrical power according to the condition “a” is provided in acondition that the electrical power of the heaters according to thecondition “b” is provided and the oscillation at the oscillationwavelength β is achieved. Therefore, the oscillation wavelength is notchanged to the oscillation wavelength α. This is because the oscillationcondition is on the overlapping condition. It is therefore necessary toprovide an electrical power of the heaters according to the condition“a′” in order to achieve the oscillation at the oscillation wavelengthα. The condition “a′” is a temperature condition where the oscillationwavelength α doe not overlap with another one. Therefore, thetemperature of the condition “a′” is higher than that of the condition“a”. Accordingly, it is necessary to set the temperature of the heatersin an area where each terrace does not overlap with each other, when theoscillation wavelength is changed to an adjacent one.

The overlapping of each terrace has a correlation with an interval ofeach oscillation wavelength (that is, an interval of each wavelengthchannel). When the interval of each oscillation wavelength is narrow, anoverlapping range gets broader. Therefore, a condition for changing theoscillation wavelength to an adjacent one (that is, a condition whereterraces do not overlap with each other) gets narrower when the intervalof the oscillation wavelength is reduced.

On the other hand, the inventor has researched a difference of a terracecaused by a changing direction of the temperature of the heater. FIG. 2Billustrates the difference of the terrace caused by the changingdirection of the temperature of the heater in FIG. 2A. In FIG. 2B, acircle indicates a changing of an oscillation wavelength of a case wherethe temperature of the heaters 30 a through 30 c is changed from a lowerone to a higher one in a single direction. In FIG. 2B, a triangleindicates a changing of the oscillation wavelength of a case where thetemperature of the heaters 30 a through 30 c is changed from a higherone to a lower one in a single direction.

As illustrated in FIG. 2B, an overlapping is not observed in atemperature of the heater for changing the oscillation wavelength to anadjacent one, when the changing direction of temperature is fixed toonly one of an increasing direction and a descending direction.Therefore, a terrace range for changing the oscillation wavelength to anadjacent one is enlarged. Based on the research, a description will begiven of the embodiment.

FIG. 3 illustrates a control flowchart for describing the firstembodiment. FIG. 4 illustrates a setting value of a wavelength selectionportion in a case where the control flowchart of FIG. 3 is executed. Inthe embodiment, knowledge of the present invention is adapted to a casewhere an oscillation wavelength of the wavelength-tunable laser 100 ischanged from a first wavelength to a second wavelength.

The controller 60 sets a setting value for achieving an oscillation atthe first wavelength on the wavelength-tunable laser 100. A settingvalue for achieving the oscillation at the first wavelength is set onthe gain electrode 40, the heaters 30 a through 30 c, and thetemperature control device 50 of the wavelength-tunable laser 100. Thesetting value is read from the memory 70. Temperature setting value ofthe heaters 30 a through 30 c for achieving each oscillation wavelengthis set in a temperature condition where each terrace does not overlapwith each other.

The wavelength-tunable laser 100 laser-oscillates when the settingvalues are set on each element. The oscillation wavelength obtained bythe laser oscillation is a wavelength that is output because ofoverlapping of the wavelength characteristics of the SG-DFB region 10and the wavelength characteristics of the CSG-DBR region 20. Theoverlapping allows a wavelength range. The wavelength-tunable laser 100oscillates at any wavelength in the wavelength range.

The controller 60 calculates a difference between wavelength informationobtained through the beam splitter 80 and the wavelength detectionportion 90 and a target wavelength. The temperature control device 50uses the calculated wavelength difference information. The temperaturecontrol device 50 varies refractive index of the resonator according tothe temperature variation. Thus, the temperature control device 50 usesthe characteristics and moves the oscillation wavelength to the targetwavelength. The operation is called AFC (Auto Frequency Control).

With the control, the wavelength-tunable laser 100 oscillates at thefirst wavelength (Step S1). In Step S1, a setting value is set on theheaters 30 a through 30 c in order to output the first wavelength. Thecondition is a control point “a” of FIG. 4.

Next, a command structure gives an instruction so that the oscillationwavelength is changed to the second wavelength in Step S2 of FIG. 3.Next, a changing direction from the first wavelength to the secondwavelength is confirmed in Step S3 of FIG. 3. For example, when thesecond wavelength is a wavelength “B” of FIG. 4, the temperature of theheaters 30 a through 30 c is increased to the point “b”. This changingdirection is defined as a changing direction “plus”. On the contrary,when the second wavelength is a wavelength “C” of FIG. 4, thetemperature of the heaters 30 a through 30 c are decreased to the point“c”. This changing direction is defined as a changing direction “minus”.

Next, it is determined whether the control direction confirmed in StepS3 corresponds to a setting direction (changing direction) determined inadvance (Step S4). As mentioned above, when the changing direction oftemperature is a single direction, the wavelength characteristics do notoverlap with each other. And so, in the embodiment, the changingdirection of the temperature is determined to one of an increasingdirection and a decreasing direction in advance. In the embodiment, thechanging direction of the temperature is the increasing direction (thechanging direction “plus”).

If the changing direction confirmed in Step S3 is the changing direction“plus”, the changing direction confirmed in Step S3 is the same as thedirection determined in advance. Accordingly, Step S6 is executed. InStep S6, the temperature of the heaters 30 a through 30 c are increasedto the point “b” of FIG. 4 as a setting value for achieving the secondwavelength. With the control, the wavelength-tunable laser 100oscillates at the second wavelength (the wavelength “B”).

Next, a description will be given of a case where the changing directionconfirmed in Step S3 is the changing direction “minus”. If the changingdirection confirmed in Step S3 is the changing direction “minus”, StepS5 is executed. In Step S5, a value that is shifted from a setting valuefor achieving the objective second wavelength (the wavelength C) in adirection that is opposite to the pre-determined changing direction isset as a prepared setting value. That is, as illustrated in FIG. 4, asetting value (temperature “c′”) that is positioned in an opposite side(the changing direction “minus” that is opposite to the pre-determinedchanging direction “plus”) of a setting value (temperature “c”) forachieving the wavelength “C” is set when the second wavelength is thewavelength “C”. In FIG. 4, it is not determined whether the laseroscillation is achieved at the wavelength “C” or the wavelength “D”,because the temperature “c′” is an area where the terrace of thewavelength “C” overlaps with that of the wavelength “D”.

Next, Step S6 is executed. In Step S6, a setting value for achieving thesecond wavelength is set. Here, the temperature of the heaters 30 athrough 30 c is controlled to be the point “c”. The control direction ofthe temperature is the increasing direction (the changing direction“plus”) from the temperature “c′”. Therefore, the temperature iscontrolled in the direction corresponding to the changing directiondetermined in advance. And, the temperature is controlled to thetemperature “c” finally. As illustrated in FIG. 4, the point “c” is anarea where the terrace of the wavelength “A” and the terrace of thewavelength “C” overlap with each other. In accordance with theabove-mentioned description, the wavelength-tunable laser 100 oscillatesat the wavelength “C” certainly.

The point “c′” determined in Step S5 may be a value that is smaller thanthe target point “c”. That is, the point “c′” may be a value positionedin a direction that is an opposite of the changing direction determinedin advance. However, as illustrated in FIG. 4, when an overlapping rangeis large, it is necessary to determine the point “c′” that exceeds arange indicated with “z” in FIG. 4. In FIG. 4, the range indicated with“z” is an area where the terrace of the wavelength “C” (the secondwavelength) overlaps with the terrace of the wavelength “A” that isadjacent to the wavelength “C”.

The temperature “c′” in the embodiment overlaps with the terrace of thewavelength “D” adjacent to the wavelength “C” (the second wavelength).After that, the temperature is controlled to the point “c” in Step S6.Therefore, it is not a problem, even if the wavelength-tunable laser 100oscillates at the wavelength “D” when the point “c′” is set. It is not aproblem, even if the point “c′” is set to be further low value. Forexample, the same effect can be obtained, if the temperature of theheaters 30 a through 30 c is increased to the point “c” after thedriving current of the heaters 30 a through 30 c are set to be zero.

In accordance with the embodiment, a desirable oscillation wavelengthcan be selected with a simple structure. It is therefore possible tocontrol an oscillation wavelength with high controllability.

The embodiment may be modified. In the embodiment, the changingdirection is the increasing direction of temperature. However, the sameeffect can be obtained when the temperature is controlled in a reversedirection, if the changing direction is the decreasing direction of thetemperature. In the embodiment, the wavelength-tunable laser 100 usesthe heaters 30 a through 30 c as a wavelength selection portion.However, a wavelength-tunable laser varying refraction index with use ofcurrent injection instead of the heaters may be used.

The embodiment solves a problem of the overlapping of the terraces thatis occurred inevitably in the wavelength-tunable laser of whichwavelength characteristics has the terrace. The wavelengthcharacteristics having the terrace is occurred in a wavelength-tunablelaser having a resonator including a plurality of wavelength controlportions of which wavelength characteristics obtained at an interval.Therefore, the control in accordance with the embodiment can be adaptedto another wavelength-tunable laser.

For example, the embodiment can be adapted to a wavelength-tunable laserhaving a resonator in which a two SG-DBR mirror having corrugations atan interval are provided, a wavelength-tunable laser oscillating at adesirable wavelength with use of a combination of ring resonators, orthe like. The modification is adapted to the following second embodimentthrough the fourth embodiment.

Second Embodiment

Next, a description will be given of a second embodiment. FIG. 5Aillustrates a control flowchart for describing the second embodiment.FIG. 5B illustrates a setting value of a wavelength selection portion ina case where the control flowchart of FIG. 5A is executed. In the firstembodiment, in Steps S3 and S4, the changing direction of the settingvalue of the wavelength selection portion for oscillating at the secondwavelength is confirmed, and a control operation is switched accordingto the changing direction. In the embodiment, the changing direction isnot confirmed. The setting value is necessarily initialized when theoscillation wavelength is changed. A description will be given of anoperation of the embodiment.

The wavelength-tunable laser 100 oscillates at the first wavelength inStep S11 of FIG. 5A. In the condition, it is assumed that the heaters 30a through 30 c are controlled at the point “a” of FIG. 5B. Next, aninitial setting value is set on the wavelength selection portion in StepS13, when an instruction is given so as to change the oscillationwavelength to the second wavelength in Step S12. In the embodiment, achanging direction is determined in advance, too. In the embodiment, thechanging direction determined in advance is the temperature increasingdirection (the changing direction “plus”).

The initial setting value is an setting value that is a starting pointof the control direction determined in advance. The initial settingvalue may be a value of a terrace positioned at an end in an oppositedirection of the pre-determined changing direction in selectableterraces in the wavelength-tunable laser 100. The initial setting valuemay be a value on the opposite direction side compared to the endterrace. When the pre-determined changing direction is the increasingdirection, the electrical power provided to the heaters 30 a through 30c may set to be zero. The temperature condition of the heaters 30 athrough 30 c is the temperature condition “c′” of FIG. 5B with theexecution of Step S13, because the initial electrical power provided tothe heaters is set to be zero in the embodiment.

Next, a value for achieving the second wavelength is set on the heaters30 a through 30 c in Step S14. With the control, the temperature iscontrolled in a direction corresponding to the changing directiondetermined in advance. Thus, the temperature is controlled to the targetvalue (the point “c” or “b”) finally. It is therefore possible to obtainthe desirable wavelength (the wavelength “C” or “B”) certainly with thecontrol in accordance with the embodiment.

If the second wavelength is the wavelength “B”, the wavelength “B” canbe obtained even if the temperature is controlled from the point “a” tothe point “b” directly. However, it is not necessary to determine acondition if an initial setting value is set once in the embodiment,even if the target wavelength is anyone. The control is easier in thesecond embodiment than in the first embodiment, in this point. Thesecond embodiment may be adapted to variable wavelength-tunable lasersas well as the first embodiment.

Third Embodiment

Next, a description will be given of a third embodiment. In the firstembodiment and the second embodiment, the control direction to thesetting value for achieving the second wavelength is only a singledirection when the oscillation wavelength is changed from the firstwavelength to the second wavelength. On the other hand, in thewavelength selection portion (the SG-DFB region 10 and the CSG-DBRregion 20), the wavelength is controlled to a target value from anon-controlled value at a starting of the wavelength-tunable laser 100as well as a switching of the wavelength.

In the controlling time, the oscillation wavelength is changed from thewavelength A to the wavelength B or from the wavelength A to thewavelength C described in the first embodiment or the second embodiment.That is, setting values according to a target oscillation wavelength areset on the gain electrode 40, the heaters 30 a through 30 c and thetemperature control device 50 in the wavelength-tunable laser 100 ofFIG. 1 in order to obtain a laser oscillation.

At the starting, the wavelength is controlled to the target value fromthe non-controlled value. Each element to be controlled gets stabilizedthrough a transition period according to each condition. The transitionperiod of the temperature control device 50, the heaters 30 a through 30c and so on is changed according to an external temperature. Thetransition period is variable. Therefore, there is a case where thewavelength is controlled in an area where a terrace in the temperatureincreasing direction and a terrace in the temperature descendingdirection have an influence, in the period all controls are notstabilized. In this case, the wavelength-tunable laser 100 may oscillateat a wavelength adjacent to a target wavelength in the area whereterraces overlap with each other.

The third embodiment solves the problem. In the third embodiment, thecontrol direction for achieving a target wavelength is a singledirection, as well as the first embodiment and the second embodiment. Adescription will be given of an operation of the third embodiment. FIG.6A illustrates a control flowchart in accordance with the thirdembodiment. FIG. 6B illustrates a setting value of a wavelengthselection portion in a case where the control flowchart of FIG. 6A isexecuted.

In the embodiment, at first, a setting value of the wavelength selectionportion set an initial setting value, and don't set a target value, atthe starting of the wavelength-tunable laser 100. That is, a gainsetting value is set on the gain electrode 40, and a setting value forachieving the target wavelength is set on the temperature control device50, as illustrated in Step S21 of FIG. 6A. In this case, a setting valuefor achieving the target wavelength (the point “a” of FIG. 6B) is notset on the heaters 30 a through 30 c acting as the wavelength selectionportion, and the initial setting value is set. The temperature controldevice 50 controls the wavelength selection operation of the wavelengthselection portion (the SG-DFB region 10). In the embodiment, the initialsetting value is set on the heaters 30 a through 30 c that is one of aplurality of the wavelength selection portions.

The initial setting value of the second embodiment may be adapted to thethird embodiment. That is, the initial setting value may be a value of aterrace positioned at an end in an opposite direction of thepre-determined changing direction in selectable terraces in thewavelength-tunable laser 100 or a value on the opposite direction sidecompared to the end terrace. In the third embodiment, the changingdirection determined in advance is the temperature increasing direction(the changing direction “plus”), and the initial setting value is avalue for achieving a condition that no electrical power is provided tothe heaters 30 a through 30 c. In the condition, as illustrated in FIG.6B, the heaters 30 a through 30 c are in the temperature condition ofthe point “a-base”.

Next, a setting value for achieving a target wavelength is set on theheaters 30 a through 30 c in Step S22. That is, the temperaturecondition is controlled to the point “a” from the point “a-base”illustrated in FIG. 6B. In this case, the heaters 30 a through 30 c arecontrolled in the pre-determined direction (the changing direction“plus”), and the heaters 30 a through 30 c are not controlled in areverse direction. It is therefore possible to achieve an accurateoscillation wavelength.

Fourth Embodiment

Next, a description will be given of a fourth embodiment. The fourthembodiment is an example of a control at the starting of thewavelength-tunable laser 100 as well as the third embodiment. Adescription will be given of an operation of the fourth embodiment. FIG.7A illustrates a control flowchart for describing the fourth embodiment.FIG. 7B illustrates a setting value of the wavelength selection portionin a case where the control flowchart of FIG. 7A is executed.

In the fourth embodiment, all setting values for achieving a targetwavelength is set on the wavelength-tunable laser 100 at first, beingdifferent from the third embodiment. That is, a gain setting value isset on the gain electrode 40 in Step S31 as illustrated in FIG. 7A. Asetting value for achieving the target wavelength is set on thetemperature control device 50. A setting value for achieving the point“a”, the target wavelength, is set on the heaters 30 a through 30 c.However, it is possible that the wavelength-tunable laser 100 oscillatesat the wavelength “B” or the wavelength “C” that are adjacent to thetarget wavelength “A”, in the condition.

Next, an initial setting value is set on the heaters 30 a through 30 cin Step S32. The initial setting value may be the same as the initialsetting value of the third embodiment. In the fourth embodiment, thechanging direction is the temperature increasing direction (the changingdirection “plus”). With the control of Step S32, the initial settingvalue “a-base” is set on the heaters 30 a through 30 c in Step S32. Inthe embodiment, the initial setting value of the electrical power set onthe heaters 30 a through 30 c is zero.

Next, a setting value, that is a value according to the point “a′”(=“a”) of FIG. 7B, for achieving the target wavelength is set on theheaters 30 a through 30 c in Step S33. With the controls of Step S32 andS33, the heaters 30 a through 30 c are controlled in the pre-determineddirection (the changing direction “plus”), and are not controlled in thereverse direction. This allows an accurate oscillation wavelength.

As mentioned in the above embodiments, the present invention allows anoscillation wavelength with high controllability by determining acontrol direction for achieving a target wavelength to a single one.

The present invention is not limited to the specifically disclosedembodiments and variations but may include other embodiments andvariations without departing from the scope of the present invention.

1. A method of controlling a wavelength-tunable laser selecting anoscillation wavelength with a combination of a plurality of wavelengthselection portions of which wavelength peak is different from eachother, in a case where a setting value of at least one of the wavelengthselection portions is controlled in a process in which the oscillationwavelength is changed from a first wavelength to a second wavelength,comprising: a first step of changing a first setting value for achievingthe first wavelength set on the wavelength selection portion to aninitial setting value acting as a starting point of a control directiondetermined in advance; and a second step of changing the initial settingvalue set on the wavelength selection portion in the first step to asecond setting value for achieving the second wavelength.
 2. The methodas claimed in claim 1, wherein the initial setting value is a value sothat the wavelength selection portion is not controlled.
 3. The methodas claimed in claim 1, wherein at least one of the plurality of thewavelength selection portions is comprised by a CSG-DBR that has anoptical waveguide in which corrugations are formed at an interval, atleast two of the intervals being different from each other.
 4. Themethod as claimed in claim 3, wherein at least one of the plurality ofthe wavelength selection portions is comprised by a SG-DFB that has anoptical waveguide in which corrugations are formed at an interval, theintervals being substantially equal to each other.
 5. The method asclaimed in claim 4, wherein a wavelength property of the CSG-DBR iscontrolled by a heater.
 6. The method as claimed in claim 5, wherein:the wavelength-tunable laser has a wavelength property of terraces thatkeeps an oscillation wavelength of the wavelength-tunable lasersubstantially constant with respect to a temperature variation of theheater; and a position of the terraces are different from each otherwith respect to a changing direction of the temperature variation.